Ever since Hurricane Ivan battered the island in September, there has been a raging debate about the future of the Palisadoes peninsula, the main access-way to the Norman Manley International Airport, and historic Port Royal which is earmarked for redevelopment.

The government is now mulling over suggestions that the airport should be relocated to Vernamfield in Clarendon because its current location is a disaster-prone area, and there is no space for runway extension.

Professor Edward Robinson from the University of the West Indies’ Marine Geology Unit in the Department of Geography and Geology has done extensive research on the issue of the Palisadoes and has submitted the following article:

ON the night of September 10th, 2004, Hurricane Ivan passed close to the south coast of Jamaica, leaving behind a trail of destruction not seen in the island since the passage of Hurricane Gilbert, almost exactly 16 years before.

One of the more significant destructive aspects of Ivan was the storm surge generated along the south coast of the island. It caused the sea level to rise as much as two metres above the highest it usually comes, and perhaps more in places.

Early Saturday morning, the fury of the storm surge became evident, when it was realised that the road to the airport along the Palisadoes was impassable.

Ivan had blocked the road with immense quantities of sand and debris. The sand had also formed, what would later be called the “Palisadoes New Beach”, new strips of land on the Kingston Harbour side up to 50 metres wide.

The stretch of road between Harbour View and the airport is on the narrowest part of the Palisadoes. Before the storm, it was partly protected by sand dunes about two metres high.

Now, since the storm, the dunes’ average height above sea level is probably less than a metre. Had Ivan delivered a more severe blow, it is possible that the sea would have broken right across the road to form a channel from the open sea side through to the harbour.

Although beach systems gradually recover naturally from such storm events, this part of the Palisadoes is in a precarious state, having lost much of the sand and rocks that protected it from the sea.

If another category five storm like Ivan were to hit us soon, say this year, would this part of the Palisadoes be able to resist its onslaught, or would it be breached by the sea? Will the airport and Port Royal become separated from the rest of Jamaica to form an island, the island of Palisadoes?

Palisadoes Island

To hazard a guess, pardon the pun, at what will happen to the Palisadoes tomorrow, one needs to know how it got to the stage it is at today.

As is well known, The Palisadoes is a strip of land, some 14 km long, that almost completely encloses Kingston Harbour. Scientists have called it both a spit and a tombolo.

A spit is a long, narrow bit of land made up of beach sediment carried by the current along the shoreline beyond a point where the coastline orientation changes abruptly. One end of a spit is attached to the shore and the other sticks out into the sea.

A tombolo is a spit of sand linking an island to the mainland or to another island, usually forming on the sheltered side of the island. I prefer to call Palisadoes a spit complex as it is evident that more than one spit has existed in the past.

No one is sure when the Palisadoes first originated. In its present form, it probably dates back some 4,000 years to the time when the present sea level was more or less established, following the ending of the last ice age, some 12,000 years ago.

Historical records show that Port Royal was once an island totally cut off from Jamaica. The island of Port Royal and other cays (little islands) were probably linked together and to mainland Jamaica by a series of spits to form what is now the Palisadoes.

Southeast stretches of the Palisadoes (such as in front of the Norman Manley airport, Plumb Point and Rocky Point), could possibly be zones covering former cays. We have no direct evidence, however, that this is the case.

What moves the beach sediment?

Beaches, and the spits their sediments help to construct, are dynamic systems in which the beach sediment – be it sand or pebbles – is in a continuous state of movement in reaction to the force of breaking waves, currents near the shore and the wind blowing over the upper, dry part of the beach.

The dominant waves approaching the Palisadoes come from the southeast, resulting in a current which moves the beach sediment gradually westward along the shore. This gradual movement of sediment from east to west does not occur at a steady pace.

During stormy periods, the energy of large waves breaking on the beach is able to move large-sized pebbles and boulders, whereas in times of calm seas, longshore movement of sand predominates and the whole process of transport along the shore will be slower.

On stretches such as that fronting the airport, the waves approach the beach and carry sediment in and out at right angles to the shore. Along here, too, the wind has generated sand dunes that were five to six metres high before Ivan.What is the source of the sediment?

At the Harbour View end of the Palisadoes, beach materials must be replaced by supplies of sediment coming even further from the east. If the Palisadoes were not replenished by sediment, it would become detached from the mainland, getting smaller and smaller through progressive loss of its eastern end, and eventually cease to exist.

The Hope and Cane Rivers, and possibly other sources further east, bring sand and gravel down to the coast where they are added to the beach system and are gradually moved west to the Palisadoes. Landslides from human-induced denudation of the hillsides add to the sediment load.

The Hope and Cane Rivers are seasonal and the sediment only gets to the Palisadoes when they are in flood and even then most of the sediment goes out to sea.

Some of the sediment is removed by mining before it gets to the Palisadoes. Apart from the rare, catastrophic hurricane events, the coastline is subject to a variety of weather systems that generate waves of varying size and energy at the beaches, as well as rainfall of varying intensity and duration over the river basins.

In some years, the weather conditions may be such that beaches will accumulate more sediment and become more extensive. In other years, weather conditions may be stormier, on average, leading to erosion.

These factors of varying sediment supply alternating cycles of calm and stormy weather in the zone nearest to the sediment source, ie the eastern Palisadoes, and could lead to conditions in which sediment supply to this part of the beach system might be severely curtailed over an extended period.

Rare, destructive events

These are mainly tropical cyclones, with earthquakes playing a locally important, but infrequent role.

From the point of view of damage to Port Royal and the Palisadoes, the worst earthquakes in historical times have been that of 1692, which destroyed much of Port Royal, with largely unknown effects on the rest of the Palisadoes, and the well-documented event of 1907.

Photographs taken after the 1907 earthquake show damage, due to liquefaction effects, to buildings at Port Royal and fissuring of the ground at the eastern end of the Palisadoes, near to where the Donald Quarrie school now stands. In 1907, part of the beach system at Port Royal collapsed into the sea.

The worst hurricane on record appears to be that of 1722. It lasted 14 hours, raised a reportedly five-metre storm surge in Port Royal, made that town a temporary island again, and created five breaches or channels through the eastern part of the Palisadoes.

On a map of Palisadoes made by Gascoigne in 1728, the locations of these channels can be matched closely with current topographic and bathymetric maps of the Palisadoes. The undersea depths on the harbour side show anomalies that I interpret to be the remnants of sediments deposited by surges through the channels, in much the same way as happened with Ivan.

Despite their widths, the four easternmost breaches from the 1722 hurricane were healed within a few years and certainly by about 1750. By contrast, the western breach persisted, was reopened on two subsequent occasions, is today one of the narrowest parts of the Palisadoes, and sports a groyne field which is supposed to protect this spot.

The wave action and rate of longshore transport in this corner of the spit probably include components that favour erosion. Overall, the groynes along here seem to have helped in minimising erosion problems, although they received a hard blow from Ivan.

Tsunami threat to the Palisadoes

We have all seen the harrowing pictures of the tsunami disaster that has hit the Indian Ocean region. Does the threat of tsunami exist for Jamaica and the Palisadoes? It does, although the likelihood of an event of the magnitude of the Indian Ocean one is very small. There are several records of tsunami that have occurred in the Caribbean.

A tsunami can be likened to a hurricane storm surge in some ways, except that it is a sudden, unexpected, very short-lived but potentially very destructive surge consisting of several waves breaking in quick succession.

The warning time potentially available for a tsunami originating in the Caribbean, be it resulting from an earthquake, an exploding volcano, a submarine landslide, or even a small meteorite splashing down in the sea, would range from a few minutes to as much as two hours, depending on location.

There is presently no warning system in place. Its effect on the Palisadoes would be the creation of washovers, similar to those from Ivan, in a severe case, perhaps also the creation of channels across the narrow part of the spit.

Rising sea level

It is now widely accepted that sea level has been rising gradually over the past century, and will continue to do so at an increasing rate in the future. A rise of at least half a metre, perhaps much more, will occur by the end of this century. How significant will this rise be for the Palisadoes, where the ground elevations are frequently less than half a metre?

Because the beach sediments of the spit are continually being moved around by the sea, it is more than likely that the beach would rise in elevation to keep pace with sea level.

On the other hand, areas that have been paved over, like the Palisadoes highway, and the airport runways, do not have the natural means to cope with a sea level rise and would be flooded. Man would need to intervene to raise the levels of these periodically.

Will the Palisadoes become an island again?

Here the story becomes entirely speculative.

A glance at the map of Palisadoes shows that the shape of the spit at Port Royal mimics the probable shapes of the spit at former positions in its development, at Plumb Point, Little Plumb Point and Rocky Point. Does this mean that future extension of Palisadoes will try to take a northwesterly course?

Perhaps this is what was beginning to happen before the disaster of the 1692 earthquake caused the northwest section of Port Royal to slip below the sea.

On the other hand, the spit can only extend so far before the currents associated with tidal flow in and out of the harbour, and the flow of the Rio Cobre prevent further westerly extension. I suspect that Port Royal is the end of the line for this particular development of the complex.

As I have suggested, in the past, new spit extensions have been to the seaward of the existing spit when a change in direction of development occurred. Each time this happened, the new spit captured the sediment supply from the previous one, which gradually degraded to form the banks of mangrove stands in the harbour today.

Perhaps future developments will generate a spit complex embracing some of the cays now outside Kingston Harbour. It may be that Gun Cay will be the next candidate for incorporation into the Palisadoes complex.